Studies will continue on the purification and characterization of erythrogenin from renal as well as extrarenal (spleen, liver) sources. further separation of subcellular components containing erythrogenin will be attempted through procedures involving centrifugation in a continuous sucrose gradient established in the zonal rotor. Investigations are also planned on the possible presence of erythrogenin in extrarenal organs (spleen, liver) of neonatal animals and of fetuses subjected to hypoxia. Further purification of the leukocytosis-inducing factor (LIF) in the plasma of rats rendered leukopenic by anti-leukocyte serum will be attempted utilizing ethanol precipitation and chromatographic procedures. Studies will be extended on the influence exerted by the LIF and the colony stimulating factor (CSF) upon leukocyte release and production employing the intact animal, perfused isolated hind leg preparations and soft agar culture techniques. It is hoped that these experiments will help to clarify the problem whether leukocyte discharge is an obligatory prelude to leukopoiesis in the normal state. The possibility that an alteration in the production of erythropoietin (Ep) is involved in the development of erythroleukemia in mice infected with Rauscher leukemia virus will be explored through the use of agents that increase endogenous production of Ep.